One of Stephen Hawking’s most famous and charismatic ideas was that the universe had no boundaries and time had no beginning.
However, it emerged yesterday that he had turned his back on the theory in the last paragraph of physics he wrote before his death.
The Cambridge University cosmologist, who died in March at the age of 76, has revised his signature hypothesis about the first instant of the Big Bang in a final, posthumous paper.
Hawking’s “no-boundary” proposal, set out jointly in 1983 with the American physicist James Hartle, held that the universe was effectively eternal because time curved back on itself like a sphere. It became so closely associated with Hawking that it furnished a pivotal scene in his 2014 biopic, The Theory of Everything, which turned the theory into a metaphor for his physical determination and courage.
It also left him unsatisfied, because it implied the possible existence of a large number of different universes, each with its own laws of physics. This made it impossible to explain why our universe should behave the way it does.
Hawking’s last scientific work, written with Thomas Hertog, a theoretical physicist at the University of Leuven in Belgium and his collaborator of two decades’ standing, announces a “significant departure” from the theory.
Professor Hertog journeyed frequently to Cambridge to develop the idea, even as it became increasingly difficult for Hawking to communicate. In fact, Hawking’s thinking had come full circle as he returned to the research of his youth, Professor Hertog said.
“There is a single, overarching question behind all of this work by Hawking,” he said. “It is a deeper understanding of where the laws of physics that we test in our labs come from.”
As a young scientist, shortly after he was diagnosed with motor neurone disease, Hawking argued that the Big Bang began with a point beyond the reach of mathematics, where the equations Einstein had coined to describe reality broke down. This was replaced by the no-boundary proposal, his first big breakthrough after he was appointed to the Lucasian chair of mathematics at Cambridge.
The new paper, published in the Journal of High Energy Physics, ditches that hypothesis and states that there is a boundary after all. It also makes for a simpler set of alternative universes.
“The idea of the ‘no boundary’ is gone again,” Professor Hertog said. “At some point 13.8 billion years in the past we had a boundary, where our familiar notion of time ceases to be meaningful and we are left with a kind of timeless state. there is nothing. No space. No time. Absolutely nothing.”
Marika Taylor, who studied under Hawking and is now professor of theoretical physics at the University of Southampton, said the no-boundary proposal had been a “beautiful” but flawed idea. Hawking’s final theory was a logical evolution of his earlier work and addressed a question to which there is still no conclusive answer, she added.
“I think it’s fair to say this is something which is still at the frontier of study. People believe that there was an inflationary period. What started that off is something that people are still exploring.”
However, Professor Taylor said that Hawking’s last paper was too speculative to ignite a revolution in cosmology: “There’s a core of sharp, rigorous work out there relating gravity to a theory with one less dimension, and they are pushing things beyond that. This is an interesting paper, and it’s Stephen’s last paper, but it’s not a breakthrough.”
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The theory explained
The no-boundary proposal was one of Stephen Hawking’s most brilliant and troubling ideas. In 1983 he published a paper that appeared to solve the origin of time itself: it had no beginning. The further back you delve, the more time simply dwindles away. He compared it to travelling to the South Pole. By the time you arrive, the word “south” has lost its meaning. However, the model had an awkward implication. It gave rise to any number of possible universes, most of them hostile to the stable existence of the matter that makes up stars, planets and human beings. Hawking found all this deeply messy. His last paper is an attempt to resolve the problem. Hawking and Thomas Hertog describe a new boundary at the start of a finite universe, which leads logically to reality as we know it.
The Times